The tensile membrane
A four-point saddle of fabric after Frei Otto: two masts lift high points, two anchors hold low ones, and the double curvature keeps the prestressed membrane stiff.
The rules it illustrates
A membrane must be a saddle. A fabric or cable-net roof is stable only if it is prestressed into an anticlastic (saddle) shape, curved upward in one direction and downward in the other, so that wind from either side meets a surface already in tension.
Where it appears in the book
In the styles High-Tech.
In the chapters Frame and Skin.